Substituent effects on the photoisomerization of 5(E)-styryl-1,3-dimethyluracil

Abstract

Fluorescence quantum yields of 5(E)-styryl-1, 3-dimethyluracils (4``-X-5(E)-SDU``s) are more affected by the solvent viscosity than by its polarity except 5 (E)-(4``-nitrostyryl)-1,3-Dimethyluracil (4``-NO$_2$-5(E)-SDU). The fluorescence maxima shift slightly to the long wavelengh with increasing the solvent polarity indicating that the lowest excited fluorescent states are the 1($\pi,\;\pi^\ast$) states. However, the lowest singlet state of 4``-NO$_2$-5(E)-SDU has (n,$\pi^\ast$) character in non-polar solvents, but with increasing the solvent polarity the lowest singlet state changes from ($\pi,\;\pi^\ast$) to ($\pi,\;\pi^\ast$) state. Relatively weak fluorescence of the substituted 5 (E)-SDU``s implies the significant contribution of the ($\pi,\;\pi^\ast$) state introduced by the heteroatom to the lowest excited singlet state leading to the fast internal conversion or intersystem crossing. Photoisomerization quantum yields of 4``-halogen substitution are similarly influenced by solvent viscosity as the unsubstituted 5(E)-SDU while little affected by solvent polarity. However, photoisomerization quantum yields of 4``-OCH$_3$-and 4``-CH$_3$-5(E)-SDU``s depend on the nature of the solvents, while for 4``-nitro derivative it is strongly affected by solvent polarity. The azulene quenching experiments on the direct and sensitized photoisomerization of the substituted 5(E)-SDU``s indicate that singlet mechanism for the direct photoisomerization of 4``-OCH$_3$-and 4``-CH$_3$-5(E)-SDU``s is operative, and both singlet and triplet mechanism for 4``-halogen substituted 5(E)-SDU``s are operative. However, direct photoisomerization of 4``-NO$_2$-SDU occurs in the triplet manifold.